A typical, currently-used, pressurized-water reactor comprises a pressure vessel having a vertically substantially cylindrical side wall having an inside; and a vertically substantially cylindrical core barrel having an outside; the vessel's inside and the core barrel's outside being radially interspaced and forming therebetween an annular, coolant descent space; the vessel's side wall having a plurality of coolant inlet openings interspaced circumferentially with respect to the side wall and through which openings coolant normally flows into the descent space. This coolant comes from the cold legs of main coolant loops of the reactor.
The top end of the core barrel has an external flange above such coolant inlet openings and which rests on an internal flange provided on the vessel's inside, this arrangement supporting the core barrel and also closing the top of the annular descent space. The core barrel contains the reactor core and various equipment above the core, the introduced coolant flowing downwardly through the descent space and upwardly through the inside of the core barrel the latter having a plurality of coolant outlet connections which open from the inside of the core barrel, span the radial descent space and connect with the hot legs of the main coolant loops. The outlet flows are, therefore, separated from the inlet flows throughout the downward extend of the annular descent space.
In the event of a break in one of the cold legs, the coolant in the vessel, being under high pressure, can rapidly discharge backwardly through the inlet opening for the broken cold leg, thus possibly emptying the pressure vessel from the coolant and resulting in a dangerous situation unless the core is scrammed and an emergency core cooling system can be operated immediately.
With the above in mind, U.S. Braun et al application Ser. No. 577,874, filed May 15, 1975, discloses a check valve assembly for the inlet opening of each of the cold legs, the assembly being positioned on the inside of the pressure vessel in the radially restricted descent space between the vessel's inside and the core barrel's outside. To provide a coolant flow area of adequate size, each assembly comprises a plurality of individually swinging flap valves and means for mounting these flap valves in the form of a series encircling the coolant inlet opening, and, of course, so that the coolant flow through the opening must pass through these flap valves. The flap valves are normally held open by inward flow but snapping shut to outward flow, thus protecting the pressure vessel against excessive loss of coolant in the event of a break in the main coolant loop, feeding that inlet opening. Because each flap valve is of small surface area and overall dimensions, each flap valve is individually light in weight, this having the advantage that in the event of a break in the cold leg, the valves snap shut with extreme rapidity.
However, it has been found that in case of a break coolant flows in the descent space at the circumferentially interspaced locations for the various coolant inlet openings, tends to be a random and ambiguous flow in the vicinities of the valve assemblies and that due to the great sensitivity of the flap valves, some of the flap valves of the valve assemblies, can possibly tend to flutter between their normal full open positions and partially or wholly closed positions.
The object of the present invention is to solve the problem presented by the above flap-valve fluttering possibility.